A step-for-step main-group replica of the Fischer carbene synthesis at a borylene carbonyl

The Fischer carbene synthesis, involving the conversion of a transition metal (TM)-bound CO ligand to a carbene ligand of the form [=C(OR’)R] (R, R’ = organyl groups), is one of the seminal reactions in the history of organometallic chemistry. Carbonyl complexes of p-block elements, of the form [E(CO)n] (E = main-group fragment), are much less abundant than their TM cousins; this scarcity and the general instability of low-valent p-block species means that replicating the historical reactions of TM carbonyls is often very difficult. Here we present a step-for-step replica of the Fischer carbene synthesis at a borylene carbonyl involving nucleophilic attack at the carbonyl carbon followed by electrophilic quenching at the resultant acylate oxygen atom. These reactions provide borylene acylates and alkoxy-/silyloxy-substituted alkylideneboranes, main-group analogues of the archetypal transition metal acylate and Fischer carbene families, respectively. When either the incoming electrophile or the boron center has a modest steric profile, the electrophile instead attacks at the boron atom, leading to carbene-stabilized acylboranes – boron analogues of the well-known transition metal acyl complexes. These results constitute faithful main-group replicas of a number of historical organometallic processes and pave the way to further advances in the field of main-group metallomimetics.

borylenes 15 ). A noteworthy achievement in this regard is our binding, reduction (and in some cases also coupling) of dinitrogen mediated by a transient, superambiphilic borylene species 16-18 . Despite the achievements in the chemistry of these "maingroup metallomimetics", the replication of organometallic reactions by p-block species is mostly limited to the field of small-molecule activation, and many archetypal organometallic processes still have no main-group counterparts. This is likely due to both: (a) the generally difficult synthesis and high reactivity of low-valent p-block species, and (b) limitations of the main-group metallomimetic concept itself. In other words, despite their similarities, main-group elements still have more limited oxidation state and orbital flexibility than TMs. One of the most promising families of boron metallomimetics has been the borylene monoand dicarbonyls, of the form [L 2-n RB(CO) n ] (n = 1, 2; L = neutral Lewis base) [19][20][21][22][23][24] , which have shown the propensity to undergo photodecarbonylation and ligand exchange, both of which are well-established processes of TM carbonyl complexes.
These reactions prompted us to question whether borylene carbonyls could serve as metallomimetic scaffolds for replicating other classical reactivity patterns of TM carbonyl complexes. We envisaged that a carbene-stabilized borylene carbonyl ([LRB(CO)], L = carbene donor) might allow us to demonstrate a main-group analogue of the Fischer carbene synthesis protocol (Fig. 1a). A further promising sign was our recent discovery that, under reducing conditions, a borylene carbonyl undergoes intramolecular migration of an aryl group to its carbonyl carbon atom, decomposing the carbene unit but furnishing a "bora-Fischer carbene" species 23 . In this work, we present the controlled nucleophilic attack at the carbonyl carbon atom of a borylene carbonyl, followed by quenching of the resulting borylene acylate with an electrophile, leading to alkoxy-/silyloxysubstituted alkylideneboranes. Varying the electrophile in certain cases leads to attack at the boron atom and the isolation of carbene adducts of acylboranesboron analogues of the well-known transition-metal acyl complexes. Together, the reactions demonstrate main-group analogues of three of the key elemental reactions in the chemistry of TM carbonyl chemistry.

Results and discussion
Step one: nucleophilic attack at a carbonyl ligand The first step of the historical Fischer carbene synthesis 1,2 is the nucleophilic attack at the carbon atom of a metal-bound CO ligand, forming an anionic complex with an acylate ligand ([L n M(C(O)R)] -) (Fig. 1a). This step is essentially a variant of the first step of another foundational organometallic reaction from 1931, the Hieber base reaction, in which a hydroxide anion (OH -) attacks at the carbon atom of [Fe(CO) 5 ], forming the anionic acylate complex [(OC) 5 Fe-C(O)OH] -25 . Our attempts to recreate this reaction centered on two borylene carbonyls stabilized by a cyclic (alkyl)(amino)carbene (CAAC; 1-(2,6-diisopropylphenyl)−3,3,5,5-tetramethylpyrrolidin-2-ylidene), namely [(CAAC)BDur(CO)] (1a; Dur = 2,3,5,6-tetramethylphenyl) and [(CAAC)BH(CO)] (1b, present as a ca. 1:1 mixture of E/Z isomers 24 . Treatment of 1a,b with the aryl nucleophiles phenyllithium and mesityllithium, respectively, provided highly reactive red solids 2a,b (Fig. 1b). Complex 2a could only be characterized by its 11 B NMR spectrum and a single-crystal X-ray diffraction (SCXRD) study that provided poor data suitable for establishment of its connectivity. However, complex 2b was fully characterized by NMR spectroscopy ( 1 H, 7 Li, 11 B, 13 C), high-resolution mass spectrometry (HRMS) and SCXRD. Their solid-state structures indicated that complexes 2a,b were indeed the desired borylene acylate species resulting from the nucleophilic attack at the positively charged boron atom of complexes 1a,b (for ADCH of 1b partial charges see Supplementary Fig. 28), which crystallize as their lithium-bound dimers (Fig. 2a). In both compounds, the OCBCN segments are only slightly distorted from planar and suggest a high degree of π-delocalization in this unit. This delocalization is reflected in the fact that the distances between the boron atom and its attached carbon atoms are very similar (2a: B-C CAAC 1.510(8), 1.496(9) Å; B-C acyl 1.520(7), 1.532(8) Å; 2b: B-C CAAC 1.507(6), 1.508(6) Å; B-C acyl 1.502(5), 1.494(5) Å). Resonances in the 11 B NMR spectra of 2a,b were found at relatively high field (2a: 24.0 ppm; 2b: 10.8 ppm), suggesting relatively high electron density at the boron atom. However, these are still significantly downfield of the resonances of their respective precursor borylene carbonyls (1a: −13.4 ppm 21 ; 1b: -23.9, -24.5 ppm 24 ), which are electron-rich, lowvalent boron species. To our knowledge, only one other such borylene acylate species is known in the literature, a potassium-bound dimer reported by our group, which is generated by an unusual reduction / aryl migration process from a borylene carbonyl 23 . This compound has an analogous structure to 2a,b, with a delocalized OCBCN unit, B-C CAAC and B-C acyl distances of 1.503(2) and 1.518(2) Å, and an 11 B NMR resonance at 15.0 ppm.
Step two: electrophilic quench of borylene acylates  26,27 . Of these two options, the Fischer carbene pathway is generally favored when: (a) the metal center is relatively electron poor (i.e., the metal does not compete with the acylate oxygen in terms of nucleophilicity), (b) the electrophile is sterically bulky and may struggle to approach the crowded metal site, and/or (c) the electrophile is relatively "hard" and thus a good match for an acylate oxygen nucleophile.
Astonishingly, for each of the two borylene acylate species 2a,b, both the reaction pathways described above (electrophilic attack at acylate oxygen or "metal") were found to be possible by employing different electrophiles. Treatment of 2a with methyl triflate, and 2b with trimethylsilyl chloride, led to isolation of 3a (53% yield) and 3b (82% yield), respectively, as red solids (Fig. 1b). Their 11 B NMR spectroscopic resonances (3a: 19.0 ppm; 3b: 15.3 ppm) are found in a similar region to those of their acylate precursors, the former having moved slightly upfield and the latter slightly downfield; likely a consequence of the disparate nature of the two electrophiles (i.e., carbonvs. silicon-centered). In their solid-state structures ( and suggesting that they are best described as CAAC-stabilized alkoxy-/silyloxyalkylideneboranes, or alternatively "bora-Fischer carbenes". These spectroscopic and structural data are in line with those of the three previously reported CAAC-stabilized alkylideneboranes bearing hydroxy, alkoxy or silyloxy groups at the alkylidene carbon atom 23 , the 11 B NMR resonances of which are in the range 15-18 ppm, and which also feature more B=C(OE)R double-bond character (B=C(OE)R 1.47-1.48 Å).
The presence of both Fischer-type and heterocyclic carbene units bound to boron in the complexes provides a rare opportunity to directly compare these two ligand types 28 . Based on DFT calculations, the nature of the bonding in compounds 3a,b can be described according to Fig. 3, whereby both carbenes are σdonors to the boron, and the boron atom in turn provides π backdonation. The Mayer bond orders (MBOs) within compound 3b suggest that there is strong π delocalization in the CBC unit, but that the B=C Fischer bond (MBO: 1.31) is slightly stronger than the B=C CAAC bond (MBO: 1.31). Similarly, the HOMO comprises the CBC π interaction and suggests that the B=C π bonding is slightly stronger to the Fischer carbene than the CAAC unit.
Protonation of in-situ-generated acylate complex 2a with triethylammonium chloride ([HNEt 3 ]Cl) provided acylborane adduct 4a as an orange solid (36% yield), while treatment of 2b with ethyl bromide provided the analogous 4b as its pale yellow complex with lithium bromide, [LiBr(4b) 2 ] (82% yield) (Figs. 1b, 2c). Attempts to sequester the LiBr by dissolving the compound in non-polar solvents such as benzene and hexane in order to obtain pure 4b were unsuccessful. Both acylborane adducts showed doublet resonances in their 11 Fig. 1b (e.g., the alkyl electrophile attacks at the acyl oxygen of 2a but the boron atom of 2b, etc.). Therefore, the electrophilic quenching reactions in Fig. 1b appear to be under steric control, as for each borylene acylate the smaller electrophile attacks at the far more crowded boron site while the larger electrophile attacks at the less crowded oxygen site. It should be noted that many more electrophilic quench reactions were attempted with a variety of electrophiles; however, only those shown in Fig. 1b led to tractable products, preventing us from obtaining a more general overview of the factors underpinning the regioselectivity. In addition, in each quench reaction shown, the presented product (either of type 3 or 4) was the sole product in the reaction mixture at full consumption of the precursor, and the alternative isomer was not detected.
In conclusion, we present herein a step-for-step main-group analogue of the Fischer carbene synthesis, resulting in carbene-stabilized alkoxy-/silyloxy-substituted alkylideneboranes, which can be considered boron analogues of conventional, transition-metal Fischer carbene complexes. Alternatively, by varying the electrophilic reagent in the synthesis, boron analogues of archetypal TM acyl complexes can be prepared. These reactions extend the use of p-block species as mimics of transition metals beyond small-molecule activation by replicating multi-step syntheses that are of historical and pedagogical importance in traditional, transition-metal-based organometallic chemistry. This work also opens up diverse opportunities in developing further metallomimetic chemistry using low-valent boron species, in particular replicating well-known reactions of TM Fischer carbenes, e.g., the Dötz reaction, olefin metathesis, or the synthesis of main- group analogues of Fischer carbynes. Work in our laboratories is underway with these goals in mind.

General synthetic considerations
All manipulations were performed either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Deuterated solvents were dried over molecular sieves and degassed by three freeze-pump-thaw cycles prior to use. All other solvents were distilled and degassed from appropriate drying agents. Solvents were stored under argon over activated 4 Å molecular sieves. NMR spectra were acquired on a Bruker Avance 400 or 500 NMR spectrometer. Chemical shifts (δ) are provided in ppm and internally referenced to the carbon nuclei ( 3 J HH = 6.5 Hz, 3H, CH(CH 3 ) 2 ) ppm. 13   Single-crystal X-ray diffraction . The structures were solved using the intrinsic phasing method 31 , refined with the ShelXL program 32 and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized positions, except the boron-bound hydrogens, which were detected in the difference Fourier map and freely refined, except when stated otherwise in the refinement details.

Computational details
Geometry optimizations were performed in the gas phase for 2b and Li[(OC) 5 W(C(O)Ph)] using the B3LYP 33-36 functional with the D3 37 dispersion correction combined with the def2-SVP 38 basis set. All optimized structures were verified as minimum through vibrational frequency calculations, showing no imaginary frequencies. The electronic situation of the compounds was investigated using ADCH 39 charges. To support the obtained results for compound 2b, CHelpG 40 charges were also calculated and are found to be in good agreement. For Li[(OC) 5 W(C(O)Ph)], only ADCH was applied. All geometry optimizations and frequency calculations were performed with the Gaussian 16, Revision C.01, program 41 . The ADCH and CHelpG charges as well as the Mayer bond orders were obtained using Multiwfn 3.8 program 42 .

Data availability
The NMR and computational data generated in this study are provided in the Supplementary